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5. Zoomass Harvests
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5 Zoomass Harvests Terrestrial trophic pyramids are invariably broadly based, with phytomass (primary producers) being commonly 20 times more abundant than the mass of herbivores (primary consumers), and the zoomass in the highest trophic level (this may be the third level in the simplest ecosystems, and only a few terrestrial communities go beyond the fifth level) may be equal to just 0.001% of the phytomass. These realities limit the mass of heterotrophs that could be profitably hunted (or collected): many small herbivores (insects, rodents) are not worth the effort, while the largest carnivores are too scarce and too dangerous to hunt. The situation is reversed in the ocean as phytoplankton’s short life span limits the total amount of standing autotrophic biomass and results in inverted trophic pyramids, with the aggregate heterotrophic biomass being commonly twice, and up to four times, as large as the biomass of primary producers and with some fish and marine mammals being fairly massive. There are no universally valid rules as far as energy transfers between successive trophic levels are concerned. Lindeman’s (1942) pioneering work on conversion efficiencies in Wisconsin’s Lake Mendota found that the primary consumers incorporated nearly 9%, secondary consumers a bit over 5%, and tertiary consumers about 13% of all energy that was available at the previous trophic level—and this led to the often cited conclusion that typical energy transfers between successive trophic levels (ecological efficiencies) average about 10%. Subsequent research disproved this simplistic conclusion, as studies in aquatic ecosystems showed transfer efficiencies as high as 40% and as low as 2%, but the10% rule is still frequently invoked. Endothermy exacts its high metabolic cost, as only 1%–3% of ingested phytomass is converted to new zoomass by birds and mammals; in contrast, many ectotherms invest roughly an order of magnitude more of their energy intake (10%–25%) 52 Chapter 5 In 1557, Pieter van der Heyden engraved Pieter Brueghel the Elder’s illustration of the proverb “big fish eat little fish.” This picaresque image illustrates both a long tradition of harvesting marine zoomass and the increasing sizes of species at the top of the trophic pyramid. to produce new zoomass (Golley 1968; Humphreys 1979; Currie and Fritz 1993). With respective means at 2% and 15%, an even split of the consumed phytomass (assuming 10% of 55 Gt C of the global net primary production, or NPP) would result in net herbivore production of some 470 Mt C/year—while Whittaker and Likens (1973) calculated what they considered to be a high estimate of about 370 Mt C/year. Global meat production (carcass weight) in 2010 was about 290 Mt; reverse application of typical live weight/dressed carcass factors (Smil 2000) yields at least 450 Mt of slaughtered live weight, or at least 90 Mt C. The annual slaughter of domestic animals would thus account for 20–25% of net secondary production. Animal domestication in general, and modern mass-scale commercial breeding and production in particular, pose a different set of energetic challenges. Little can be done about changing the metabolic efficiencies of individual species (for example, pigs are inherently more efficient converters of feed than cattle), but higher productivities can be achieved by supplying adequate amounts of optimally selected feeds. As a result, modern varieties of meat-producing heterotrophs mature faster and reach higher slaughter weights sooner than the traditional breeds and require less [44.200.23.133] Project MUSE (2024-03-29 02:42 GMT) Zoomass Harvests 53 feed per unit of edible mass than their unimproved predecessors. But these improvements could not prevent a massive increase in the total biomass required to feed the domesticated animals: because of enormous increases in their numbers, better feeding efficiencies could only reduce the rate of increase in the overall claim. Hunting and Domestication Hunting opportunities are fundamentally determined by the efficiency of energy transfer between trophic levels. Shares of the NPP that are actually consumed by herbivores are only 1%–2% in deciduous temperate forests, may surpass 25% in temperate meadows and wetlands, and reach maxima of 50%–60% in rich tropical grasslands; in the ocean they can peak at more than 95% in some patches of phytoplankton (Crawley 1983; Valiela 1984; Chapin, Matson, and Mooney 2002). Terrestrial rates between 5% and 10% are perhaps most common, implying annual transfers of roughly 3–6 Gt C. But when the calculation is restricted to vertebrates, the primary targets of hunting, it is mostly just around 1% (around 0.5...